This invention relates to an antilock control method for vehicle wheel brakes.
When the brakes of a vehicle are applied, a braking force is generated between the wheel and the road surface that is dependent upon various parameters which include the road surface condition and the amount of slip between the wheel and the road surface. This braking force increases as slip increases until a critical value of slip is surpassed. Beyond the critical value of slip, the braking force decreases and the wheel rapidly approaches lockup. Therefore, to achieve stable braking, an antilock control system seeks to operate wheel slip at or near the critical slip value. An antilock control system achieves this objective by detecting an incipient lock condition. Upon detecting an incipient lock condition, the antilock control system releases pressure at the wheel brake to allow recovery from the incipient lock condition. Once the wheel recovers from the incipient lock condition, brake pressure is reapplied. Criteria used to indicate an incipient lock condition includes excessive wheel deceleration and/or excessive wheel slip.
One known antilock control system uses a motor driven pressure modulator in which a DC torque motor drives a piston in a cylinder whose volume is modulated to control the hydraulic brake pressure at the wheel brake. In this system, because of the relationship between motor current, motor torque and motor load represented by the hydraulic brake pressure on the head of the piston, the value of motor current is used as a representation of brake pressure and is controlled to provide control of the brake pressure. In general, when an incipient wheel lock condition is sensed, the value of motor current at this time is stored as a representation of the brake pressure producing the maximum braking force coexisting with the critical slip between the wheel and the road surface and the motor current is controlled to quickly retract the piston to release brake pressure to allow recovery from the incipient wheel lock condition. When a recovery from the incipient wheel lock condition is sensed, the motor current is controlled to extend the piston to reapply brake pressure. In reapplying the brake pressure, the pressure is quickly established substantially at the brake pressure producing the maximum braking force by quickly establishing the motor current at a significant fraction of the motor current stored at the time an incipient wheel lock condition was sensed. Thereafter, brake pressure is ramped at a controlled rate that is a function of wheel slip and acceleration by ramping the motor current until an incipient wheel lock condition is again sensed after which the cycle is repeated. In general, the ramp rate is decreased with increasing wheel slip and wheel deceleration so that the ramp rate is smaller as the wheel approaches an incipient wheel lock condition. This lower ramp rate prevents an overshoot of the brake pressure resulting from system inertia when an incipient wheel lock condition is sensed.
In the foregoing form of motor driven pressure modulator, the following dynamic relationships exist: (a) when the brake pressure load on the motor is equal to the motor torque, the motor does not rotate, the piston remains stationary, and motor current is a measure of the brake pressure and (b) when the brake pressure load on the motor is small compared to the motor torque, the motor accelerates and rotates at a high rate and the piston travels at a high speed. In this latter situation, the speed of the motor is unknown and the motor current is not a true indicator of brake pressure. If this condition exists when the wheel slip approaches the critical slip, the high motor/piston speed may cause the brake pressure to overshoot the pressure producing the critical slip and will result in storing a current when an incipient wheel lock condition is sensed that represents a brake pressure other than the pressure producing the maximum braking force.
Excessive speed resulting in the brake pressure overshooting the pressure producing the critical slip may also occur when the ramp rate of the motor current is decreased in response to increasing slip and/or deceleration as the wheel slip approaches the critical slip value. At the time the current ramp rate is decreased, the motor speed is related to the prior higher ramp rate and the motor current is not a true measure of the actual pressure. If the critical wheel slip is reached soon after a decrease in the current ramp rate, the excessive motor speed may result in the brake pressure overshooting the pressure producing the critical slip and in the storing of a current representing a brake pressure other than the pressure producing the maximum braking force.
Another characteristic of the foregoing form of motor driven pressure modulator is that its compliance varies with load on the piston. When the motor load is low (i.e., low pressure present on the piston head), the motor position change necessary to create a change in pressure is greater compared to the motor position change required to produce a change in pressure when the motor load is high (i.e., piston head pressure high). Thus, for a given motor current ramp rate during application of brake pressure, the real motor speed will actually be higher at lower pressures than it will be at higher pressures. When beginning a pressure reapply from a low pressure value, if motor torque is allowed to increase at a very high rate, the motor travels at an effective speed higher than is actually desired, which may cause the system to overshoot the desired pressure.
Thus from the foregoing, it can be seen that when controlling motor current to control re-application of brake pressure following recovery from an incipient wheel lock condition, it is desirable to provide controlled movement of the motor such that the motor does not exhibit an overspeed condition.